1. Field of the Invention
This invention relates to an electronic thermostat for generating a temperature dependent control signal employing a digital, programmable memory time-temperature.
2. Prior Art
Thermostats which act to generate an electrical control signal for a furnace, air conditioning unit, or the like, typically employ a bimetal element which assumes a mechanical position dependent upon the ambient temperature. The desired temperature may be set into the device by moving an electrical contact with respect to the bimetal so that an output signal is provided when the ambient temperature reaches the set point. To allow this set point to be automatically varied over a 24-hour cycle, thermostats are available which incorporate clocks which mechanically adjust the thermostat set points at different times. When used in a residence to control a furnace this enables the temperature to be lowered during the night and at other times when the occupants are not normally home. These program devices typically allow a 24-hour period to be divided into two blocks of controllable length, and allow a different temperature setting for each block.
In view of the cost and reliability advantages of electronic circuitry, temperature sensing devices which employ an electronic element having electrical characteristics which vary as a function of temperature, such as a thermistor, are often employed in industrial devices. When used as thermostats these devices are relatively complicated since they require circuitry for generating an analog voltage proportional to the set point and a comparator for receiving this analog voltage and the output of the temperature sensitive device. While it would be possible to add a 24-hour programming clock to this form of analog thermostat, the provision of a clock mechanism would probably obviate the advantages of the electronic temperature sensing elements over mechanical elements.
Independent of these developments in thermostats great cost reductions in digital electrical circuitry have taken place in recent years through the development of integrated circuits incorporating large numbers of digital devices on a single semi-conductor chip. Once the initial tooling costs for a specific integrated circuit have been amortized large quantities of complicated circuits may be produced at very low costs. These advances have been possible devices such as low cost electronic calculators and digital watches.
The present invention is broadly directed toward an electronic thermostat employing digital circuitry so that it may be mass-produced in the form of a low cost integrated circuit. Thermostats formed in accordance with the invention enjoy advantages over conventional thermostats in both cost and reliability and provide a variety of auxiliary functions which would be extremely expensive to implement in conventional mechanical or analog electronic thermostats.